


byleth's baby sister

by knoir (bchemicalromance)



Category: Fire Emblem Series, Fire Emblem: Fuukasetsugetsu | Fire Emblem: Three Houses
Genre: ?????, Alternate Universe, Fluff and Angst, Pre-Time Skip, Sibling AU, Spoilers, in which byleth's calm and cool demeanor has been ravished bc his sister can't not embarrass him, no i could not think of a title, pls look past it and read this anyway, pre-release
Language: English
Status: Completed
Published: 2019-07-21
Updated: 2019-07-21
Packaged: 2020-07-09 01:50:39
Rating: Teen And Up Audiences
Warnings: Major Character Death, No Archive Warnings Apply
Chapters: 1
Words: 4,442
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/19879636
Author URL: https://archiveofourown.org/users/bchemicalromance/pseuds/knoir
Summary: some scenes regarding byleth and his little sister after her arrival to the monastery.





	byleth's baby sister

**Author's Note:**

> heavily motivated by [vixenofthemist on tumblr](https://vixenofthemist.tumblr.com/) and [their sibling au ideas](https://vixenofthemist.tumblr.com/tagged/fe%3A3h-sibling-au)

_Byleth,_  
_I hope all is well in the center of Fódlan, that your students haven't died, and that you are doing well._  
_Since you've left, your sister has become more than a handful. I will never understand how you managed to look after her as long as you did while still growing into the talented merc you've become._  
_The road has been harder on all of us since you've left. We have had too many close calls, and even lost some men along the way. Rest assured, your sister and I are fine, but I worry for her safety. She remains capable, but she will be much safer with you._  
_Alois will be dropping her off to the Academy in due time, I have already let Rhea know of her arrival._  
_Do not let anything happen to her under your watch._  
_Your father,_  
_Jeralt_

Byleth read the letter once more, after having finished his cup of coffee. His younger sister was always different compared to him, even though he had raised her, campaign after campaign. He taught her everything he knew about the sword, all of which he learned from their father. His sister knew how to hold her ground within battle, but between Byleth, their father, and his band of mercenaries, she never really had to. 

"Before I dismiss you all," Byleth said to his class after completing his lesson. They paused in their tracks. "I want to inform you that a new student will be joining the house."

Felix groaned. "Not another Golden Deer, Professor, please."

"No," Byleth shook his head. "My sister."

The class took to that, even Felix.

"You never told us you had a sister, Professor," Ingrid said through a wide grin. 

"No," Byleth agreed, "I have not." He dusted the chalk off of his gloves. "Anyway, expect her in the near future. You are dismissed."

The words that usually sent his students in motion kept them all uncharacteristically still. They instead looked expectantly at their Professor, as if they were listening for the due date of their latest homework assignment.

"You're not going to tell us what she's like?" Annette asked, gripping the desk and leaning forward in her chair. "Her favorite food? Anything?"

Mercedes put a hand on her friend's shoulder, and Annette relaxed in her chair. "At least tell us her name, Professor," Mercedes suggested, causing the class to rally behind her in gentle agreement.

"Her name is Bylese," Byleth said. "You are all dismissed."

The class exchanged looks with one another, before making the silent agreement to give up. In the month they have known their professor, they believed they had him all figured out. His quiet manner and stoic expression was the result of years as a mercenary, turning him into the cold shell of the man he was now. That was all there was to it. 

"Don't talk about the Professor like that," Dimitri scolded Felix during lunch. He and the others stopped counting how many times Felix complained about their new professor only a week into his employment. "He only shares what he wishes to share with us, and we don't need to know any more."

"He didn't tell us he had a sister until he had to!" Felix pressed on. "For the Goddess' sake, how are we meant to trust him?"

"Need I remind you he was invited by the Archbishop herself, Felix. Not only that, he saved mine, Edelgard, and Claude's lives," Dimitri said, not giving his friend the satisfaction of letting himself get angry. "I trust him just fine."

"Of course you do, you stupid boar," Felix snapped, slamming his elbow on the table and pointing his finger in Dimitri's face. "I'm leaving," he then declared, rising from his seat. "I'm going to look for Professor."

Ingrid started to rise from her seat to chase after him, but Dimitri gently grabbed her arm. 

"Don't bother," he said. "He's just going to demand our Professor to spar with him. Save your energy for when he loses." 

Ingrid laughed and settled into her seat once more. "If you insist, milord."

While Felix started for the classroom in search of his teacher, Byleth was at the entrance of the monastery, waiting for his sister to arrive. And eventually she did, walking beside Alois.

When Byleth had returned from his battles in the past, he would almost always be met with Bylese jumping up to hug him. For the first time, she did not do this. Instead, they both walked to each other. Bylese, who was usually bouncing up and down at the sight of her brother alive, had her arms crossed, scowling. 

"Hello, brother."

"Sister," Byleth nodded sharply, taking in his sister. She hadn't gotten any taller since the last time he saw her, but he did notice the change in her apparel. She had abandoned the knit top that she wore under her armor, exposing her chest and naked arms against armor. Her tights were now adorned in holes and sloppy patterns. He wrinkled his nose in disgust. "What happened to your clothes?"

"This?" Bylese looked down at herself, before looking back at her brother with big, mocking eyes. "It's a new look! Don't you like it?"

"No," Bylelth said, "not at all actually. It is severely impractical." He gestured dramatically to her expose abdomen. "You could get really hurt if someone gets you there. And--" he turned his attention to her arms, wincing at the thought of all the chafing that would happen from it. "That can't be comfortable."

Bylese just shrugged. "I like it just fine."

Byleth rubbed the junction of his nose and eyebrows, wondering why he missed his sister. "It doesn't matter anyway. You are a student now, you shall be given a uniform to wear."

"Uniform?" Bylese gasped, though her brother showed no sympathy.

"Yes, a uniform," Byleth repeated. "Now let's go."

Despite Bylese's earlier attitude, she managed to smile and nod when she met Rhea. She even laughed along when Manuela came in and gushed over how cute she was. Byleth supposed Bylese spent her life surrounded mostly by men, so this had to have been a welcome change. Maybe it would be better for her after all. 

"Professor Byleth!" Manuela called his attention, his sister trailing behind her. "What do you think?"

Bylese's uniform wasn't different than the rest, save for the silver accents instead of gold. Her collar was reminiscent of the one on her armor, but a smaller brooch was pinned on it this time. The purple dagger the siblings shared was swapped for a sheath fitting a full sword. While Byleth did not say anything about this matter, he thought her skirt was too short, even with the cape that touched the back of her knees. 

"It's missing one thing," Byleth mused, before reaching into his cloak and pulling out a pink headband. "This is for you."

This made Bylese smile, the way she did when her brother came back from battle, and hugged him tightly. She squeezed him harder and muffled a 'thank you' that was barely audible to her brother, but he knew. 

When she was ready, she pulled away from him and took her new accessory out of her brother's hands, placing it on her head. She blindly mussed her hair for a few moments, then looked up at her brother. She was the only person that could decipher his emotions, so she knew her hair looked fine when he nodded at it.

* * *

"This is your room," Byleth announced, opening the door to a formerly unoccupied dorm. Bylese walked in hesitantly, looking around at the four walls, the bed pushed up against one of them, and the desk against another. 

"This is mine?" Bylese asked in disbelief. 

"I understand it can be daunting," Byleth said, "it was for me too. But you'll get used to it." He took Bylese's old clothes out of her hands. "This can go here," he opened a drawer and shoved everything in, "until you wear it again."

"Okay," Bylese said absentmindedly, walking over to her bed and sitting on it. 

"Another girl who lives here, Flayn, made you some nightdresses, can you recall the last time you wore those?" Byleth asked, a smile creeping on his face. He could; it was when she was four and Byleth didn't have any more clothes he used to wear that Bylese already didn't. "Anyway, they're in here too."

He turned around to face his sister, who already had fallen asleep on her new bed. 

Later in the evening, Byleth's work was interrupted by heavy rapping on his door. He opened it cautiously, not used to getting visitors to his quarters at such a late hour. 

"It's just me, doofus," his sister said, pushing the door open enough to let herself in. "Thanks for telling me where you room was, by the way." She plopped on Byleth's bed, and it was then he noticed she brought her pillow with her. "I knocked on three doors before I found yours."

"I hope you didn't wake people up, Bylese, there are nobles living here!" Byleth shook his head, focusing on a more relevant question. "Why are you here anyway?"

Bylese's shoulders rose and fell slowly. "I woke up, it was dark, I was alone."

"You were scared," Byleth clarified, still standing at his door. His sister glared at him. 

"Was not!" She protested. "I didn't know where you were." She felt tears well up in her eyes again, as if her body felt she hadn't cried enough when she woke up in a foreign room. "You didn't even say goodbye, you know," she continued, moving her gaze to the ceiling. "I already thought the worse when you and Father took longer than usual to return back to camp. But when you didn't come back, I..." She paused to take a breath. Her inhale was shaky, and she exhaled a sob. Byleth teetered on his heels before walking over to his sister and pulling her into a hug. Calming his sister down wasn't something Byleth had to do often, but he was still better at it than their father. Even if she spent all of her live on the battlefield, sleeping in tents and sometimes inns, she was still a child. Not that she held it against either of them. They were both trying. 

"I'm sorry," he said, "you know I would if I could. Everything just happened so fast."

Bylese nodded against her brother's chest, then pulled away from him entirely. She sniffed and wiped the tears from her face. "How did you even become a teacher in the first place?"

And then they both laughed, Byleth playfully pushing his sister's shoulder. He sat back at his desk to continue his work, letting Bylese talk to him about everything he missed while he was gone. 

"Don't get too comfortable," he warned, "you should go back to your room soon."

"Nope," Bylese said, "I'm staying here."

Byleth turned to face her, finding his sister under his covers, the pillow she brought in with her under her head. "Bylese," he whined. "You have your own room now, this isn't like the tents we used to share!" 

"Your bed is more comfortable than mine," she insisted, smiling at her brother's annoyance. 

"But they're the same--" Byleth cut himself off and rubbed his temples. "Fine," he said, sitting back in his desk chair, where he would be for the night. "Do not get used to this."

He turned the chair against the wall and kicked his feet up on his desk, letting his head loll over. He awoke the next morning with a stiff neck and sore joints, and tried to figure out when he let his sister push him over like that. Maybe it was her doe-eyes. It always reminded him of their mother. 

He looked over at his still sleeping sister, who did not stir at the popping of Byleth's bones as he stretched them. He shook her shoulder. "Get up. Class starts in an hour."

Byleth was relieved when his sister entered through the double doors of the Blue Lions classroom. He trusted her to go off on her own, but her directional skills had always been severely lacking. 

"Perfect timing," he said as she shut the door behind her. He pointed to the desk up front, closest to him. "Sit there."

And she did, smoothing down the skirt of her uniform as she did. Her brother dropped books on her desk. 

"I'll bring you up to speed on everything they already learned," Byleth explained. "If you have questions, raise your hand before asking them." They heard sounds of students looming near. "Don't embarrass me," he added quickly. Bylese grinned, "no promises."

"Bylese!" Byleth insisted, before standing upright at the sound of the door opening. Bylese turned her head so she could see the students file in, realizing in that moment she had never been around kids her own age before. Much less school, for that matter. 

"Professor!" Felix boomed. "Where were you after class yesterday?"

"I apologize if you were looking for me," Byleth said, crossing his arms behind his back. Bylese covered her mouth and laughed into it. Her brother had always been a stick-in-the-mud, sure. But when did he start talking like _that?_

"My sister arrived," he gestured to Bylese, and the students followed his hand to her. 

"Wow, Professor!" Sylvain gushed. "You didn't tell us your sister was so-" he was cut off by Dimitri's elbow to his ribs. "So, uh, similar... looking to you."

"I think Sylvain is just surprised at her sudden arrival," Dimitri corrected, leaving his friend to rub at the sore spot where he'd been hit. "After all, it was just yesterday you told us of her arrival in the 'near future'."

"Well, yes," Byleth agreed, "her arrival was most certainly earlier than expected. But no matter, as she is here now to learn with the rest of you." With that, he picked up a textbook from his desk. "Now take your seats so I can start."

Byleth taught his class as usual, though when he dismissed them, everyone gravitated to Bylese. 

"How old are you?" Ashe asked. "Teach refuses to tell us his age."

Byleth rolled his eyes. "Because my age isn't of importance."

"I'm eighteen," Bylese said. This time, Byleth scoffed. 

"No you are not," he retorted. "In seven months, maybe."

Bylese glared at Byleth, to which Byleth glared back at Bylese. The class observed this interaction, unsure of what to make of it. They had seen Byleth glare before, of course they have. Whenever Felix demanded to spar or when Sylvain talked back. But it was never like this. 

"Say," Ingrid piped in, putting a halt to the sibling's competition. "Why don't you join us for lunch, Bylese? We'd all love to learn more about you."

Byleth was never one to tease, but he would remember the face his sister made was perfect material for the next time they squabbled. The way her head bobbed up and down, and how she cranked her voice up an octave before saying 'Okay!' was simply priceless. 

"That is alright with you, Professor, isn't it?" Ingrid then asked. It was then Byleth realized he was smiling, and he quickly dropped it in favor of his usual frown. It was too late though, as everyone, even his sister, saw it. 

"Yes, that's fine," he responded, trying to ignore some of his student's shell-shocked faces. "Now go," he shooed them off. "We'll meet in the Drill Grounds later. Don't be late."

As Byleth's students piled out with Bylese in tow, he heard them all bombard her with questions that did not cease even when they all settled at their table with their food. 

"What's it like being Teach's sister?"

"Does he know how old he is?"

"What's up with him and fishing?"

"This is a wild idea," Sylvain butted in, "but maybe we can ask the pretty lady one thing at a time?"

"Sylvain..." Dimitri said warningly, but to his surprise, Ingrid came to his defense. 

"No, Sylvain is actually right this time. Our apologies."

"It's fine," Bylese managed. "He knows how old he is."

The table perked up at that, their forks, spoons, and goblets pausing in mid air, turning their attention to Bylese. 

"Well?" Dimitri said. "How old is he?"

Bylese looked at the eager faces around her. She could tell them, or...

"He's old," she settled, watching as their expressions twitched. 

"How old is old?" Annette asked. Bylese brought her goblet to her mouth. 

"Old enough."

Bylese drank from her goblet as her new classmates groaned.

* * *

"What do you think of the Monastery?" Sylvain asked as the Blue Lions walked to the Drill Grounds. Since the House didn't separate during their lunch break, they all walked to the latter half of their day together. Sylvain noticed Bylese was falling behind, and stepped out of pace without his friends noticing. 

"It's...different." Bylese said, tearing her gaze away from her surroundings to look at him. "I've never been anywhere like this before."

"Or even remotely close to it, I bet," he added, and they both laughed. 

"Yeah," she breathed, "pretty much."

"I'm sure you've seen a lot, though," he continued. "Being a mercenary and all that?"

"I wouldn't call myself a mercenary," she shook her head. "I just traveled with them, and they traveled a lot. It feels weird staying in one place for more than a week."

"You'll get used to it," he reassured. "Not only do you have your brother here, but you also have a bunch of new friends! We've all helped each other out at one point."

"Will you too hurry up?" Felix shouted at them. Bylese and Sylvain looked up from each other to find their class had already arrived, Felix and Dimitri being the only one's outside waiting for them. 

Byleth was stood in the center of the grounds with his hands clasped behind him as he waited for the last of his students to make their way in. Bylese never remembered him like that. He always stood with his hands at his sides or one hand ready to sheath his sword. Never, in all of Bylese's life, has she ever seen her brother look so important. Even when they followed their father and his mercenaries, Byleth was never in charge. It was always 'Captain Jeralt', his secoond commander Alois, a few more of Jeralt's twisted men, and then Byleth. Their father never questioned the abilities of either of his kids, they suppose he would rather see them when it wasn't life or death. 

"Today we are continuing to practice the sword," Byleth announced, ignoring Annette's small groan. "Grab a sword and figure out who will spar first."

"Say, Teach," Sylvain said, being the first to grab a wooden sword from the cart beside their teacher. He handed it to Bylese before getting another for himself. "Why don't you and Bylese show us how it's done?"

"No," Byleth and Bylese said in unison. Byleth cleared his throat and continued. "We learned from the same person," he said, "it wouldn't be much of a learning experience for you all to just watch a mirror image."

"That's fine by me," Felix said, pointing to Bylese. "We're fighting. Now."

The class by now was well accustomed to Felix's demands, and, while usually by force, he got what he wanted. Dimitri and Ingrid sighed as they made their way to sit on the sidelines, the rest of the house following in suit. Mercedes and Annette sat a bit closer to the battle than the rest of their classmates, ready to heal if needed. And it was today of all other days, where they thought it would be needed. 

Byleth couldn't help but feel hesitation for this mock battle. Maybe it was fear either one of them would get hurt, or being unable to guess which one of them would come victorious at the end. He stood between the two as they prepared. 

Felix, you know the rules already," he said, turning to his sister.

"Don't hit his face, abdomen, or groin," he told her. "Please, Bylese, don't. He is a noble and bears a Major Crest."

Byleth dropped her sword arm, cocking her head to the side. "I don't know what that means."

"Neither do I," Byleth whispered. "Just be gentle, alright? He's not a bandit or myself or Father."

"Okay," Bylese groaned. "Can I fight now?"

* * *

"One more time!" Felix shouted, picking himself off the ground for the third time. "I was just warming up!" 

"Could I spar with someone else?" Bylese offered, turning to the class, who all looked less than willing.

"Maybe this is a good time to stop," Byleth said. "Write a half page report about the spar you saw today, highlighting at least three strengths and weaknesses from each party." He turned to Felix and Bylese. "You two shall write a half page personal reflection about your session, while nothing what you could have done better. Class dismissed."

While his students didn't do anything besides watch for the past hour, they all rose from their spots as if they had sparred with each other all afternoon. 

"Byleth," his sister said, tugging at his sleeve. "I have to do that too?"

"Yes," Byleth deadpanned, pulling his arm free from his sister's grasp. "You are a student."

Bylese than proceeded to stare at her brother, in hopes that if she did long enough, he would change his mind. This has never once worked, but Bylese thought it would still be worth a shot. 

"Why don't you let me help you?" Sylvain offered. "We can motivate each other to actually do it on time!"

Byleth walked out of the Grounds before he could hear his sister's answer, but when dinner time rolled around he wondered of his sister's whereabouts. He made way for her quarters first, as he didn't imagine she had any need to be somewhere like the library. He opened the door to her room, more than surprised to find his sister and his student sitting on her bed, making out. 

Byleth often forgot that his sister was only a teenager. A teenager who spent all of her life traveling, mostly surrounded by men. The only fun she had was pranking mercenaries who were passed out drunk. She deserved that moment. 

Just not today, and certainly not with Sylvain.

He found his entrance wasn't enough, so he cleared his throat and watched the pair jump away from each other, as if they were magnets that suddenly repelled. 

"Byleth!" Bylese yelled. "I have a door now, _knock!_ "

Byleth ignored his sister, instead narrowing his eyes at Sylvain. 

"Teach," Sylvain said, holding his arms up. "I..."

Byleth just shook his head. "Get out."

"Teach," Sylvain said again, "let's talk about this."

Though Byleth did not want to talk about it. He wanted Sylvain out. So, he marched over to him, grabbed him by his lapel, and dragged him out of the room. 

The siblings argued for the rest of the night.

* * *

They had just finished a battle. 

It was tiring, to say the least. The heavy rain wore everyone down to begin with, the fog that came after hindered their sight. They had already lost their strength taking time to find the enemy, but along the way they found Jeralt and his mercenaries, which lifted a heavy weight off Byleth's shoulders. Though the hope Byleth mustered was quickly shot down. And not only was his hope shot down, but so was Ingrid, who went to see if she could fly above the fog, only to come crashing down with an arrow through her pegasus. With an easy blast of Divine Pulse, though, she was back as quickly as she left, with a warning to stay close. Two Divine Pulses later, they successfully routed the enemy, leaving everyone tired and chilled, as the rain more than soaked through their clothes. 

Byleth leaned against a tree as he watched his students treat any prominent injuries. He took the time to catch his breath and relax a little. Divine Pulse took a lot out of him, even with just one use. He never had to use it more than three times at once, but he knew for certain he wouldn't be able to command the rest of the battle after. 

He realized he was missing two important units to his army when his nausea passed. He walked over to his students, who all looked at him expectantly.

"Is something wrong, Professor?" Dimitri asked, sweeping his wet bangs out of his face. 

"Bylese." he said, looking around as though she was hiding. "And Jeralt."

Byleth's students followed him as they looked for their missing units, each one taking turns shouting out their names. As they turned a corner, Byleth put up a hand, and his students stopped in their tracks, weapons ready. 

The rain picked up by now, or Byleth noticed how heavy it pelted against his armor then. Either way, he was still able to make out Bylese's teal hair in the dark of the night. 

"Bylese," he called as he walked closer to her. As he did. he could tell that she was kneeling. Kneeling over a body. Kneeling over their father's body. He ran the rest of the way. 

"What happened?" He breathed, dropping down next to his sister. He was bale to see their father, their dead father, with blood oozing through a large slash through his stomach. "Bylese," he pleaded, "what _happened?_ "

His sister did not answer him, instead gabbed a limp hand and cried harder. 

"What happened? What did you do?"

"I didn't," Bylese choked out. "I found him like this. I don't know."

Byleth held his sister, tucking her head under his. This was exactly what their father did not want them to see.

* * *

A month later, there was a knock on Byleth's door. It was late, which easily determined who it could be. 

"Why aren't you sleeping?" Byleth asked as his sister entered his quarters. "I'm about to," she said, throwing her pillow down on Byleth's bed.

"Do you even plan to sleep in your own bed?" Byleth asked in annoyance, watching as his sister made herself comfortable. 

"I'm doing your bed a favor," she replied, "you should thank me."

"And what favor are you doing for my bed?"

"This is the only time a woman will ever be in it."

Byleth rolled his eyes. "You're disgusting."

"You love me," she said simply, turning over into a proper sleeping position.

And he did.

**Author's Note:**

> \- in the five days it took for me to finish writing this it devolved from being a chronological fic to snippets of moments  
> \- in the five days it took me to finish writing this and the two days it took me to actually get this up here i never once thought of a title.  
> \- if memory serves correct when female byleth was announced her name was first bylese? maybe? i understand it may be difficult to differentiate between byleth and bylese but  
> \- when i started writing this fic i wasn't sure if that was the case so i called her beres but i changed it a good two days into it  
> \- anyway i imagine byleth being very calm and unbothered while bylese pretends to be unbothered but she is actually very bothered. please ask her what's on her mind  
> \- headcanon that bylese used to have a shirt/tunic/something under her armor that she took off one day just cuz  
> \- headcanon that bylese also used to have plain stockings that she ripped up and maybe repatterned? i'm not sure if that's possible but she did have plain stockings at one point  
> \- headcanon that even though bylass' school uniform doesn't have the purple dagger that her's and bylad's main design have know it is somewhere on her person. probably in the waistband of her skirt, but who knows? i certainly don't  
> \- headcanon that bylese is really used to sleeping in the same space as byleth because she has for all her life  
> \- headcanon that byleth can sleep anywhere  
> \- bylese knows very well how old her brother is but decides to Not say bc she's having fun messing with her new friends  
> \- uh as i am looking back at this i see that there are a few loose ends that i never tie up so? if i really wanted to? i could make a second chapter/part two? of just siblings at the monastery? in non chronological order? if i feel so compelled to do so?  
> \- when byleth asks "what happened what did you do" it's mostly to himself because he thinks that he used divine pulse at the wrong time and could've stopped it had he noticed the disappearance of his father during battle  
> \- shoutout to the three houses ao3 page. our corner of the fandom has grown so much since e3 2018. the only reason why i actually wrote these fics instead of letting them stay in my head was that so the time between the original release date of spring 2019 to july 26 (soon!!!!) would go by faster. we've all rode the hype train, we've done it. i hope you all enjoy the game and i cannot wait to read all of your post release multi chaptered fics in the years to come (yes. years.)  
> \- also big big big shoutout to the discord server that i am in as it's been ten times more exciting to ride the hype train with all of them. thank you for the memes and the highest levels of foolishness. and the support to actually finish this fic ~~i almost let this one go unfinished~~


End file.
